Squats probably are the best single exercise (if for some reason you could only do one) but I'm sick of people saying you CANNOT gain muscle without them.... you'll be scrawny forever. Whatever.
For me it's probably dumbbell pullovers as well.... I try them every once in a while and they don't seem to do much.
I disagree about abs training, though. Look at old pictures of guys in POW camps.... they had almost no bodyfat, but only a few of them have visible abs. I put that down to the muscles not being developed enough (or having atrophied from starvation) even with the very low bodyfat. Of course you need diet to SEE abs, but ab exercises are good for making them prominent.